Some 54 years ago, a new type of vehicle – the hovercraft – first took flight and, in the summer of 1959, made its famed crossing of the English Channel. SR.N1 was an early example of an air-cushion vehicle or ACV: a craft capable of operating over land, water, swamp or ice at speed and, importantly, also while stationary.
ACVs function by creating a cushion of air between the hull of the craft and any smooth surface below. How is this air cushion generated? Propellers on top of the hovercraft, sometimes augmented by fans, force air downwards, beneath the vessel. Surrounding the base of the ACV is a flexible ‘skirt’ or ‘curtain’, which traps the air currents, keeping them under the hull.
Typically hovering at heights of between 200 mm (7.9 in) and 600 mm (24 in), ACVs tend to operate above 20 knots (23 mph, 37 km/h). Although their use has fallen out of favour commercially, they are still deployed across the world as specialised vehicles for disaster relief, coastguard, military and survey applications, and also as sports and recreational machines.
The concept of a hovering vehicle has been around for several centuries, but it took Christopher Cockerell to bring this particular version to fruition: he came up with the idea of attaching a flexible skirt to the bottom of the hovercraft to turn it into a practical mode of transportation.
In a marvellous piece of ‘kitchen sink engineering’, he studied the ring of airflow created by blowing high-pressure air into the space between two concentric tin cans – in his case a coffee can and a cat food tin! He noticed that this set-up created a ring of fast-moving air, which acted as a barrier to the air on either side of it. He dubbed it the ‘momentum curtain’ and understood that it could be used to trap high-pressure air in the area inside the curtain, in order to provide lift. Subsequent modifications to the skirt improved performance, raising the hovercraft higher, and eventually more advanced designs eventually enabled the vehicles to float over obstacles.
Build a Hovercraft at home!
We can follow in Cockerell’s footsteps and do some engineering of our own by building a
simple model of a hovercraft from just a few easily available items at home. The model is simple to construct and will only take an hour or two in total. (Note, that if using a glue gun, kids should have adult supervision.)
- An old CD or DVD
- A balloon
- A pop-top cap from a liquid soap bottle or a water bottle
- A hot glue gun
What to do
- If using the cap from a water bottle, cover the centre hole of the disk with a piece of tape.
- Poke several holes in the tape with a pin or thumbtack to slow down the airflow – this will enable the hovercraft to hover for longer.
- Use the glue gun to stick the cap to the centre of the disk, being sure to create a good seal to prevent air from escaping. Give the glue time to set.
- Decorate the disk if you like.
- Blow up the balloon and pinch the neck to hold the air in. (Don’t tie it.)
- Make sure the pop-top is closed and fit the neck of the balloon over the pop-up portion of the cap. (You may find this easier with two people.)
- Get ready to hover…. Set the hovercraft going on a smooth surface by snapping open the top.
The air flowing from the balloon lifts the CD and reduces the friction between the disk and the surface beneath it, enabling it to travel smoothly. You can experiment by trying to push the hovercraft across a table without blowing up the balloon and seeing how it performs in comparison. Try it over different surfaces – smooth, rough, water – and with different sizes of balloon.
Also, why not experiment with various shapes, sizes and materials for the body, by using corrugated cardboard or a paper plate? See how much weight the hovercraft can carry. It’s a great way to demonstrate some simple physics and have some fun at the same time.